(1) Field of the Invention
This invention relates to a pattern and method of using the pattern for the measurement of overlay accuracy of two patterns interconnected on a wafer using a photo stitch process.
(2) Description of the Preferred Embodiments
Chip sizes are often larger than the available reticle size used to expose patterns on the wafer. This requires interconnecting patterns on two different reticles using a photo stitch process. In a photo stitch process overlay accuracy is of great importance. Methods of measuring overlay accuracy and patterns used in the measurement of overlay accuracy are also important.
U.S. Pat. No. 5,701,013 to Hsia et al. describes a wafer metrology pattern integrating both overlay and critical dimension features.
U.S. Pat. No. 4,149,085 to Davis et al. describes a method and apparatus for performing automatic overlay measurements on semiconductor wafers.
U.S. Pat. No. 5,498,500 to Bae describes an overlay measurement mark and method of measuring an overlay error between multi patterns of a semiconductor device.
U.S. Pat. No. 5,300,786 to Brunner et al. describes an optical phase shift test pattern, monitoring system, and process.
As chip images become larger the circuit images become too large for a single mask reticle. In these cases it is necessary to divide the chip image into two separate reticles and expose them separately on an integrated circuit wafer. The images of each of the two separate reticles are then stitched together to make a single circuit image. Alignment of the images of the two separate reticles is of critical importance in forming a single circuit image from the two separate images. It is very desirable that the alignment of two images can be made easily using visual inspection of the images.
It is a principle objective of this invention to provide a pattern and method for determining the overlay accuracy of two images to be stitched together in a single chip.
This objective is achieved using a first overlay pattern adjacent to a first chip image on a mask and a second overlay pattern adjacent to a second chip image on a mask, preferably the same mask. When a layer of photoresist is exposed with the first and second chip images it is also exposed with the first and second overlay patterns. After the photoresist is developed, the first and second overlay patterns are visually inspected and the overlay accuracy in both the X and Y directions can be determined.
The first overlay pattern comprises a rectangular first center mark, a first number of rectangular first side marks having a width equal to a first distance to the left of the first center mark, and the first number of rectangular second side marks having a width equal to the first distance to the right of the first center mark. The second overlay pattern comprises a rectangular second center mark, the first number of rectangular third side marks to the left of the second center mark, and the first number of rectangular fourth side marks to the right of the center mark. The third side mark nearest the second center mark has a width equal to the first distance plus a second distance and the width of each successive third side mark increases by the second distance. The fourth side mark nearest the second center mark has a width equal to the first distance plus the second distance and the width of each successive fourth side mark increases by the second distance.
There is a rectangular third center mark located within the second center mark, wherein the top of the third center mark coincides with the top of the second center mark. There is a third distance between the bottom of the third center mark and the bottom of the second center mark.
The alignment accuracy in the X direction is determined by visually comparing the position of the image of the first side marks relative to the image of the third side marks and the position of the image of the second side marks relative to the image of the fourth side marks. The alignment accuracy in the Y direction is determined by visually comparing the distance between top of the image of the first center mark to the top of the image of the third side mark relative the image of the third distance.
Perfect alignment in the X direction is indicated when the right side of the image of each of the first side marks is co-linear with the right side of the image of one of the third side marks and the left side of the image of each of the second side marks is co-linear with the image of the left side of one of the fourth side marks. Perfect alignment in the Y direction is indicated when the distance between the image of the top of the first center mark and the image of the top of the third center mark is equal to the third distance.